This section provides background information related to the present disclosure which is not necessarily prior art.
In EC motors, optocouplers are used for the galvanically isolated signal transmission of useful signals for open-loop, closed-loop or monitoring functions. The galvanic isolation is used to protect individuals and material and in particular isolates the sensitive open-loop control electronics, to which an EC motor is connected, from the motor electronics of the EC motor. Inexpensive optocouplers, in which simple and inexpensive IC structures are utilised, are used in particular in EC motors for products with a high number of parts. These inexpensive variants of optocouplers consist of an optical emitter, typically a light-emitting diode (LED), and an optical receiver, in particular a phototransistor. However, these components degrade over time, depending in particular on the input current used and the ambient temperature. Degraded optocouplers cause erroneous signal transmission and thus malfunctions in the motor electronics, and this in turn can have a negative influence on other parts of the EC motor and reduce the service life thereof.
The direct current transfer ratio (CTR) of an optocoupler indicates the ratio, in percent, between the input current IF and the output current IC. It is known that the CTR decreases with a degraded optocoupler. In this regard, optocoupler manufacturers have general service life graphs which show an aging ratio as a function of the temperature and the input current. In order to determine the actual CTR of an optocoupler in an EC motor, the input current IF and the output current IC have to be measured. Owing to the measured variables being galvanically isolated and to the different ground potentials, the evaluation is only possible using complex and time- and cost-intensive measuring circuits.
In addition, the definition of end of the service life varies according to the manufacturer, with some feeling that the end of the service life is when the CTR is reduced to 50% of the start value of a new optocoupler. Normally, a very large tolerance range is given for the CTR (for example, Vishay TCLT1105 has a CTR of from 50% to 150%). The actual end of the service life of an optocoupler in an EC motor cannot be reliably established by assessing the service life graphs of the manufacturer and measuring or calculating the CTR. The actual end of the service life depends very much on how the optocoupler is wired, i.e. with respect to the motor electronics, and on the true input current and the true ambient temperatures. In the case of the end of the service life being theoretically defined as 50% of the original CTR start value of a new optocoupler, the functionality of a motor electronics wired to the output of the optocoupler can still be guaranteed in practice.